In one known method, a passive pressure limiting valve is monitored for opening. An open pressure limiting valve is detected according to a load shedding, in that the rail pressure exceeds a threshold value, subsequently a steady-state status of the combustion engine is detected, and supplementally a characteristic value of the rail pressure control loop deviates significantly from a reference value. The integral portion of the rail pressure regulator and, for example, a PWM signal for controlling the suction throttle are understood as characteristic values of the rail pressure control loop.
In a second known approach, a method is provided for monitoring a passive pressure limitation valve according to a load shedding. In a first stage, proceeding from a steady-state rail pressure, for example 1800 bar, it checks whether the rail pressure has exceeded a first, higher threshold value, for example 1850 bar. In a second stage, it then checks whether the rail pressure has exceeded a second, still higher threshold value, for example 1920 bar, despite a temporary increase in the control signal for the suction throttle. If both threshold values have been exceeded, then the pressure limiting valve is set as open. Based on the control of the pressure limiting valve, the case can indeed occur in practice that the pressure limiting valve is indeed detected as open by the evaluation program, however, it is still actually closed. The consequence is an operator false alarm and an erroneous follow-up response.
In one aspect of the second approach described above, a method is provided which checks whether the rail pressure has exceeded the second threshold value and subsequently fallen below a further threshold value having a lower pressure level than the second threshold value. Having fallen below the further threshold value, the rail pressure control deviation is then monitored for a predetermined time period. If the rail pressure control deviation is constantly greater than, for example, 20 bar during this time period, then, upon expiration of the time, the pressure limiting valve is set as open. It is critical that a pressure limiting valve can tend towards leakage once it has been opened and can cause undesired leakage during normal operation. The leakage corresponds to that fuel volume flow which discharges undesirably into the fuel tank via the pressure limiting valve. In turn, the leakage also affects a decreasing total efficiency, as the high-pressure pump must convey more fuel into the rail so that the rail pressure is achieved. In the advanced stage, the high-pressure pump can no longer maintain the set rail pressure, that means, the engine output drops and the exhaust values deteriorate with a clearly visible opacity.